Add a way to compute the base64 uri

[RCasatta]

Some downstream lib use

pub fn string_to_qr(str: &str, pixel_per_module: Option<u8>) -> Result<String, QrError> {
    let qr_code = qr_code::QrCode::new(str)?;
    let mut bmp = qr_code.to_bmp();
    if let Some(pixel_per_module) = pixel_per_module {
        bmp = bmp.add_white_border(2)?;
        if pixel_per_module > 1 {
            bmp = bmp.mul(pixel_per_module)?;
        }
    }
    let mut enc = base64::write::EncoderWriter::new(Vec::new(), &general_purpose::STANDARD);

    bmp.write(&mut enc)?;
    let delegate = enc.finish()?;

    let base64 = String::from_utf8(delegate)?;
    Ok(format!("data:image/bmp;base64,{base64}"))
}

It would be great to have a base64 feature that enable a function like this